diff --git a/bin/core/imag-annotate/src/main.rs b/bin/core/imag-annotate/src/main.rs
index 790ce8fb..d196c7cb 100644
--- a/bin/core/imag-annotate/src/main.rs
+++ b/bin/core/imag-annotate/src/main.rs
@@ -57,7 +57,6 @@ use libimagrt::runtime::Runtime;
 use libimagrt::setup::generate_runtime_setup;
 use libimagstore::store::FileLockEntry;
 use libimagstore::storeid::IntoStoreId;
-use libimagutil::warn_exit::warn_exit;
 
 mod ui;
 
@@ -75,7 +74,13 @@ fn main() {
                 "add"    => add(&rt),
                 "remove" => remove(&rt),
                 "list"   => list(&rt),
-                _        => warn_exit("No commandline call", 1)
+                other    => {
+                    debug!("Unknown command");
+                    let _ = rt.handle_unknown_subcommand("imag-annotation", other, rt.cli())
+                        .map_err_trace_exit_unwrap(1)
+                        .code()
+                        .map(std::process::exit);
+                },
             }
         });
 }
diff --git a/bin/core/imag-gps/src/main.rs b/bin/core/imag-gps/src/main.rs
index 3277547b..502497a2 100644
--- a/bin/core/imag-gps/src/main.rs
+++ b/bin/core/imag-gps/src/main.rs
@@ -53,7 +53,6 @@ use libimagentrygps::types::*;
 use libimagentrygps::entry::*;
 use libimagrt::setup::generate_runtime_setup;
 use libimagrt::runtime::Runtime;
-use libimagutil::warn_exit::warn_exit;
 use libimagerror::trace::MapErrTrace;
 use libimagerror::exit::ExitUnwrap;
 use libimagerror::io::ToExitCode;
@@ -75,7 +74,13 @@ fn main() {
                 "add"    => add(&rt),
                 "remove" => remove(&rt),
                 "get"    => get(&rt),
-                _        => warn_exit("No commandline call", 1)
+                other    => {
+                    debug!("Unknown command");
+                    let _ = rt.handle_unknown_subcommand("imag-gps", other, rt.cli())
+                        .map_err_trace_exit_unwrap(1)
+                        .code()
+                        .map(::std::process::exit);
+                }
             }
         });
 }
diff --git a/bin/core/imag-link/src/main.rs b/bin/core/imag-link/src/main.rs
index bc6af042..4dd8e0de 100644
--- a/bin/core/imag-link/src/main.rs
+++ b/bin/core/imag-link/src/main.rs
@@ -104,7 +104,13 @@ fn main() {
                 "remove" => remove_linking(&rt),
                 "unlink" => unlink(&rt),
                 "list"   => list_linkings(&rt),
-                _ => panic!("BUG"),
+                other    => {
+                    debug!("Unknown command");
+                    let _ = rt.handle_unknown_subcommand("imag-link", other, rt.cli())
+                        .map_err_trace_exit_unwrap(1)
+                        .code()
+                        .map(::std::process::exit);
+                },
             }
         })
         .or_else(|| {
diff --git a/bin/core/imag-ref/src/main.rs b/bin/core/imag-ref/src/main.rs
index d806ff95..96dedad9 100644
--- a/bin/core/imag-ref/src/main.rs
+++ b/bin/core/imag-ref/src/main.rs
@@ -67,8 +67,12 @@ fn main() {
             match name {
                 "deref"  => deref(&rt),
                 "remove" => remove(&rt),
-                _        => {
-                    debug!("Unknown command"); // More error handling
+                other => {
+                    debug!("Unknown command");
+                    let _ = rt.handle_unknown_subcommand("imag-ref", other, rt.cli())
+                        .map_err_trace_exit_unwrap(1)
+                        .code()
+                        .map(::std::process::exit);
                 },
             };
         });
diff --git a/bin/core/imag-store/src/main.rs b/bin/core/imag-store/src/main.rs
index 60f80d1c..5ec16ce6 100644
--- a/bin/core/imag-store/src/main.rs
+++ b/bin/core/imag-store/src/main.rs
@@ -50,6 +50,7 @@ extern crate libimagutil;
 extern crate libimagutil;
 
 use libimagrt::setup::generate_runtime_setup;
+use libimagerror::trace::MapErrTrace;
 
 mod create;
 mod delete;
@@ -92,9 +93,12 @@ fn main() {
             "update"   => update(&rt),
             "verify"   => verify(&rt),
             "dump"     => dump(&mut rt),
-            _ => {
+            other      => {
                 debug!("Unknown command");
-                // More error handling
+                let _ = rt.handle_unknown_subcommand("imag-store", other, rt.cli())
+                    .map_err_trace_exit_unwrap(1)
+                    .code()
+                    .map(std::process::exit);
             },
         };
     } else {
diff --git a/bin/core/imag-tag/src/main.rs b/bin/core/imag-tag/src/main.rs
index 39f468a5..200456cd 100644
--- a/bin/core/imag-tag/src/main.rs
+++ b/bin/core/imag-tag/src/main.rs
@@ -86,9 +86,12 @@ fn main() {
                 debug!("id = {:?}, add = {:?}, rem = {:?}", id, add, rem);
                 alter(&rt, id, add, rem);
             },
-            _ => {
-                error!("Unknown command");
-                ::std::process::exit(1)
+            other => {
+                debug!("Unknown command");
+                let _ = rt.handle_unknown_subcommand("imag-tag", other, rt.cli())
+                    .map_err_trace_exit_unwrap(1)
+                    .code()
+                    .map(std::process::exit);
             },
         });
 }
diff --git a/bin/domain/imag-bookmark/src/main.rs b/bin/domain/imag-bookmark/src/main.rs
index 48f349c4..3438ee78 100644
--- a/bin/domain/imag-bookmark/src/main.rs
+++ b/bin/domain/imag-bookmark/src/main.rs
@@ -77,8 +77,12 @@ fn main() {
                 "collection" => collection(&rt),
                 "list"       => list(&rt),
                 "remove"     => remove(&rt),
-                _            => {
-                    debug!("Unknown command"); // More error handling
+                other        => {
+                    debug!("Unknown command");
+                    let _ = rt.handle_unknown_subcommand("imag-bookmark", other, rt.cli())
+                        .map_err_trace_exit_unwrap(1)
+                        .code()
+                        .map(::std::process::exit);
                 },
             }
         });
diff --git a/bin/domain/imag-contact/src/main.rs b/bin/domain/imag-contact/src/main.rs
index 3c898d60..e8dda553 100644
--- a/bin/domain/imag-contact/src/main.rs
+++ b/bin/domain/imag-contact/src/main.rs
@@ -102,8 +102,12 @@ fn main() {
                 "show"   => show(&rt),
                 "find"   => find(&rt),
                 "create" => create(&rt),
-                _        => {
-                    error!("Unknown command"); // More error handling
+                other    => {
+                    debug!("Unknown command");
+                    let _ = rt.handle_unknown_subcommand("imag-contact", other, rt.cli())
+                        .map_err_trace_exit_unwrap(1)
+                        .code()
+                        .map(::std::process::exit);
                 },
             }
         });
diff --git a/bin/domain/imag-diary/src/main.rs b/bin/domain/imag-diary/src/main.rs
index ccaf3ab8..201f8431 100644
--- a/bin/domain/imag-diary/src/main.rs
+++ b/bin/domain/imag-diary/src/main.rs
@@ -48,6 +48,7 @@ extern crate libimagtimeui;
 extern crate libimagutil;
 
 use libimagrt::setup::generate_runtime_setup;
+use libimagerror::trace::MapErrTrace;
 
 mod create;
 mod delete;
@@ -80,8 +81,12 @@ fn main() {
                 "edit" => edit(&rt),
                 "list" => list(&rt),
                 "view" => view(&rt),
-                _        => {
-                    debug!("Unknown command"); // More error handling
+                other    => {
+                    debug!("Unknown command");
+                    let _ = rt.handle_unknown_subcommand("imag-diary", other, rt.cli())
+                        .map_err_trace_exit_unwrap(1)
+                        .code()
+                        .map(std::process::exit);
                 },
             }
         });
diff --git a/bin/domain/imag-habit/src/main.rs b/bin/domain/imag-habit/src/main.rs
index ccb8e71e..61b69bd6 100644
--- a/bin/domain/imag-habit/src/main.rs
+++ b/bin/domain/imag-habit/src/main.rs
@@ -90,9 +90,12 @@ fn main() {
                 "status" => today(&rt, true),
                 "show"   => show(&rt),
                 "done"   => done(&rt),
-                _        => {
-                    debug!("Unknown command"); // More error handling
-                    exit(1)
+                other    => {
+                    debug!("Unknown command");
+                    let _ = rt.handle_unknown_subcommand("imag-habit", other, rt.cli())
+                        .map_err_trace_exit_unwrap(1)
+                        .code()
+                        .map(::std::process::exit);
                 },
             }
         })
diff --git a/bin/domain/imag-log/src/main.rs b/bin/domain/imag-log/src/main.rs
index 7c2804ec..bc5a6a74 100644
--- a/bin/domain/imag-log/src/main.rs
+++ b/bin/domain/imag-log/src/main.rs
@@ -72,9 +72,12 @@ fn main() {
     if let Some(scmd) = rt.cli() .subcommand_name() {
         match scmd {
             "show" => show(&rt),
-            _        => {
-                error!("Unknown command");
-                ::std::process::exit(1)
+            other    => {
+                debug!("Unknown command");
+                let _ = rt.handle_unknown_subcommand("imag-log", other, rt.cli())
+                    .map_err_trace_exit_unwrap(1)
+                    .code()
+                    .map(std::process::exit);
             },
         }
     } else {
diff --git a/bin/domain/imag-mail/src/main.rs b/bin/domain/imag-mail/src/main.rs
index a108d692..b7d67f80 100644
--- a/bin/domain/imag-mail/src/main.rs
+++ b/bin/domain/imag-mail/src/main.rs
@@ -54,7 +54,13 @@ fn main() {
                 "import-mail" => import_mail(&rt),
                 "list"        => list(&rt),
                 "mail-store"  => mail_store(&rt),
-                _             => debug!("Unknown command") // More error handling
+                other         => {
+                    debug!("Unknown command");
+                    let _ = rt.handle_unknown_subcommand("imag-mail", other, rt.cli())
+                        .map_err_trace_exit_unwrap(1)
+                        .code()
+                        .map(std::process::exit);
+                }
             }
         });
 }
diff --git a/bin/domain/imag-notes/src/main.rs b/bin/domain/imag-notes/src/main.rs
index de395c54..6d193b4c 100644
--- a/bin/domain/imag-notes/src/main.rs
+++ b/bin/domain/imag-notes/src/main.rs
@@ -66,8 +66,12 @@ fn main() {
                 "delete" => delete(&rt),
                 "edit"   => edit(&rt),
                 "list"   => list(&rt),
-                _        => {
-                    debug!("Unknown command"); // More error handling
+                other    => {
+                    debug!("Unknown command");
+                    let _ = rt.handle_unknown_subcommand("imag-notes", other, rt.cli())
+                        .map_err_trace_exit_unwrap(1)
+                        .code()
+                        .map(std::process::exit);
                 },
             };
         });
diff --git a/bin/domain/imag-timetrack/src/main.rs b/bin/domain/imag-timetrack/src/main.rs
index ef857d54..2f432452 100644
--- a/bin/domain/imag-timetrack/src/main.rs
+++ b/bin/domain/imag-timetrack/src/main.rs
@@ -56,6 +56,7 @@ use week::week;
 use year::year;
 
 use libimagrt::setup::generate_runtime_setup;
+use libimagerror::trace::MapErrTrace;
 
 fn main() {
     let version = make_imag_version!();
@@ -77,9 +78,12 @@ fn main() {
             "track"    => track(&rt),
             "week"     => week(&rt),
             "year"     => year(&rt),
-            _ => {
-                error!("Unknown command");
-                1
+            other      => {
+                debug!("Unknown command");
+                rt.handle_unknown_subcommand("imag-timetrack", other, rt.cli())
+                    .map_err_trace_exit_unwrap(1)
+                    .code()
+                    .unwrap_or(0)
             },
         }
     } else {
diff --git a/bin/domain/imag-todo/src/main.rs b/bin/domain/imag-todo/src/main.rs
index 7eaf1d80..22ae1ede 100644
--- a/bin/domain/imag-todo/src/main.rs
+++ b/bin/domain/imag-todo/src/main.rs
@@ -51,10 +51,16 @@ fn main() {
     match rt.cli().subcommand_name() {
         Some("tw-hook") => tw_hook(&rt),
         Some("list") => list(&rt),
+        Some(other) => {
+            debug!("Unknown command");
+            let _ = rt.handle_unknown_subcommand("imag-todo", other, rt.cli())
+                .map_err_trace_exit_unwrap(1)
+                .code()
+                .map(std::process::exit);
+        }
         None => {
             warn!("No command");
         },
-        _ => unreachable!(),
     } // end match scmd
 } // end main
 
diff --git a/doc/src/09020-changelog.md b/doc/src/09020-changelog.md
index 93d16341..f4b86427 100644
--- a/doc/src/09020-changelog.md
+++ b/doc/src/09020-changelog.md
@@ -45,6 +45,9 @@ This section contains the changelog from the last release to the next release.
       commandline flag.
     * `imag-habit today --done` and `imag-habit status --done` was added for
       showing habits which are already done.
+    * `libimagrt` allows external subcommands now in the default clap app
+      builder helper. It also provides a helper for handling unknown
+      subcommands: `Runtime::handle_unknown_subcommand()`. See docs for details.
 * Minor changes
     * A license-checker was included into the CI setup, which checks whether all
       ".rs"-files have the license header at the top of the file
diff --git a/lib/core/libimagrt/src/runtime.rs b/lib/core/libimagrt/src/runtime.rs
index 8cb4521c..07b24bd4 100644
--- a/lib/core/libimagrt/src/runtime.rs
+++ b/lib/core/libimagrt/src/runtime.rs
@@ -24,6 +24,7 @@ use std::process::exit;
 use std::io::Stdin;
 
 pub use clap::App;
+use clap::AppSettings;
 use toml::Value;
 use toml_query::read::TomlValueReadExt;
 
@@ -189,6 +190,7 @@ impl<'a> Runtime<'a> {
             .version(version)
             .author("Matthias Beyer <mail@beyermatthias.de>")
             .about(about)
+            .settings(&[AppSettings::AllowExternalSubcommands])
             .arg(Arg::with_name(Runtime::arg_verbosity_name())
                 .short("v")
                 .long("verbose")
@@ -502,6 +504,81 @@ impl<'a> Runtime<'a> {
             Some(::std::io::stdin())
         }
     }
+
+    /// Helper for handling subcommands which are not available.
+    ///
+    /// # Example
+    ///
+    /// For example someone calls `imag foo bar`. If `imag-foo` is in the $PATH, but it has no
+    /// subcommand `bar`, the `imag-foo` binary is able to automatically forward the invokation to a
+    /// `imag-foo-bar` binary which might be in $PATH.
+    ///
+    /// It needs to call `Runtime::handle_unknown_subcommand` with the following parameters:
+    ///
+    /// 1. The "command" which was issued. In the example this would be `"imag-foo"`
+    /// 2. The "subcommand" which is missing: `"bar"` in the example
+    /// 3. The `ArgMatches` object from the call, so that this routine can forward all flags passed
+    ///    to the `bar` subcommand.
+    ///
+    /// # Return value
+    ///
+    /// On success, the exit status object of the `Command` invocation is returned.
+    /// On Error, a RuntimeError object is returned.
+    ///
+    /// # Details
+    ///
+    /// The `IMAG_RTP` variable is set for the child process. It is set to the current runtime path.
+    ///
+    /// Stdin, stdout and stderr are inherited to the child process.
+    ///
+    /// This function **blocks** until the child returns.
+    ///
+    pub fn handle_unknown_subcommand<S: AsRef<str>>(&self,
+                                                    command: S,
+                                                    subcommand: S,
+                                                    args: &ArgMatches)
+        -> Result<::std::process::ExitStatus, RuntimeError>
+    {
+        use std::io::Write;
+        use std::io::ErrorKind;
+
+        let rtp_str = self.rtp()
+            .to_str()
+            .map(String::from)
+            .ok_or(RuntimeErrorKind::IOError)
+            .map_err(RuntimeError::from_kind)?;
+
+        let command = format!("{}-{}", command.as_ref(), subcommand.as_ref());
+
+        let subcommand_args = args.values_of("")
+            .map(|sx| sx.map(String::from).collect())
+            .unwrap_or_else(|| vec![]);
+
+        Command::new(&command)
+            .stdin(::std::process::Stdio::inherit())
+            .stdout(::std::process::Stdio::inherit())
+            .stderr(::std::process::Stdio::inherit())
+            .args(&subcommand_args[..])
+            .env("IMAG_RTP", rtp_str)
+            .spawn()
+            .and_then(|mut c| c.wait())
+            .map_err(|e| match e.kind() {
+                ErrorKind::NotFound => {
+                    let mut out = self.stdout();
+
+                    if let Err(e) = writeln!(out, "No such command: '{}'", command) {
+                        return e;
+                    }
+                    if let Err(e) = writeln!(out, "See 'imag --help' for available subcommands") {
+                        return e;
+                    }
+
+                    e
+                },
+                _ => e,
+            })
+            .map_err(RuntimeError::from)
+    }
 }
 
 /// Exported for the `imag` command, you probably do not want to use that.